aa777888, there's no primary radar feed into FR24 in the UK. I think it might be unique to the US. However there is a slight delay with the displayed data, supposedly for safety (terrorist) reasons.
Bravo73 is closest, as FR24's main source of data is from ADS-B equipped aircraft. However they also will show an aircraft location if they can get a fix based on Multilateration from multiple receivers. However, tactical military aircraft are also filtered out whether they are ADS-B equipped or not.
FR24 have an FAQ and "How it Works" sections on their website if more info is needed.
So back to the OP, I would think that it is highly likely you were seeing a military aircraft.